EMBRYOLOGY.' 
The Quadrate Placenta of Sciurus Hudsonius ; or, 
THE Common Red Squirrel.— In 1887, the present writer 
called attention to the existence of certain vestigiary placental 
structures developed during the early stages of the mouse, rat 
and field-mouse, which indicated that the discoidal placental 
disk of the late stages of foetal life of these forms had been de- 
rived from one, the placenta of which was zonary or girdle- 
like in form, as in the cat, dog, hyrax, elephant, etc. All of 
the forms of rodents mentioned, however, possess at a late 
stage a very distinctly discoidal placenta, the development of 
which seems to be associated with the so-called inversion of the 
germ layers, which is so marked a feature of their ontogeny, 
and one also which renders its processes amongst the most 
specialized and complex known to embryologists. The notice 
published in September, 1887, as to the persistence of a girdle- 
like vestige of the decidua continuous with opposite sides of 
the placental disk, afforded only tentative evidence of the 
derivation of the discoidal placental from the zonary form. Re- 
cently some remarkably conclusive evidence, favoring such a 
view, has fallen under my observation in foetuses of the com- 
mon red squirrel. Mr, J. P. Moore, one of the pupils in the 
biological laboratory of the University of Pennsylvania, during 
the latter part of March, brought in a gravid female red squirrel 
in which two foetuses were found, in utero, which are the basis 
of the following account. 
These foetuses measured 16 mm. in length from the vertex 
of the head to the end of the body. The two cerebral vesicels 
had just appeared as a pair of smooth saccular diverticula from 
the sides of the anterior end of the neural tube. *The spinal 
cord filled out the vertebral canal entirely, and the two enlarge- 
ments, brachial and lumbar, were distinctly visible through the 
integument of the dorsal median line. The limbs were so far 
developed as to show the digits distinctly differentiated. The 
stage, in fact, represents one which is very nearly equivalent 
to that of the human embryo at three months. 
The peculiarity of the most importance in the present case, 
in relation to the question of the origin of the discoidal pla- 
centa in other forms, is the unusual shape presented by that 
organ, which is quadrate in Sciurus hudsonius. Both foetuses 
' This department is editc.l by Professor John A. Ryder, University of Pennsyl- 
